This uses a new nav style with the heirarchy:
```
Breadcrumbs
|- Title
|- Description
|- Third-Level Navigation
```
The navigation bar uses the transparent red-underlined
buttons similar to the user activity page.
Over time all admin pages will use this, but this starts
with the new plugin show page.
---------
Co-authored-by: Ella <ella.estigoy@gmail.com>
* Simplify config nav link generation to always inject the Settings
tab
* Auto-redirect to the first non-settings config link (if there is one)
when the user lands on /admin/plugins/:plugin_id
* Add `extras` to admin plugin serializer so plugins can add more
data on first load
* Add PikadayCalendar page object for system specs, extracted from the
CalendarDateTimePicker to make it more generic.